The present invention relates to optical media and, more particularly, to the fabrication of gratings within optical media. A major objective of the invention is to provide for less absorptive refractive-index gratings in optical fibers and to make such gratings easier and more economical to fabricate.
Along with the increasing prominence of the Internet has come wide-ranging demand for increased communications capabilities, including more channels and greater bandwidth per channel. Optical media, such as optical fibers, promise an economical alternative to electrical conductors for high-bandwidth long-distance communications. A typical optical fiber includes a silica core, a silica cladding, and a protective coating. The index of refraction of the core is higher than the index of refraction of the cladding to promote internal reflection of light propagating down the core.
Optical fibers can carry information encoded as optical pulses over long distances. The advantages of optical media include vastly increased data rates, lower transmission losses, lower basic cost of materials, smaller cable sizes, and almost complete immunity from stray electrical fields. Other applications for optical fibers include guiding light to awkward places (as in surgery), image guiding for remote viewing, and sensing.
The signal carrying ability of optical fibers is due in part to the capability of producing long longitudinally-uniform optical fibers. However, longitudinal variations in index of refraction, e.g., those associated with refractive-index gratings, can be included in the optical fibers to affect throughgoing pulses in useful ways. Gratings can be grouped into short-period, e.g., about 0.5 micron (.mu.m), or long-period, e.g., about 200 .mu.m, gratings. Short-period gratings can reflect incident light of a particular wavelength back on itself in the fiber. (Short-period gratings are also called Bragg gratings or Hill gratings.) Long-period gratings can couple incident light of a particular wavelength into other co-propagating modes on the fiber. Some of these other co-propagating modes may be lossy, so the overall effect of the long-period grating can be to selectively block certain wavelengths from propagating efficiently through the fiber.
While there are many methods for establishing a refractive-index grating within a fiber, the most practical methods involve exposing photosensitive fibers to patterned light. The index of refraction of certain fiber-optic materials, such as germanium-doped silica, is changed upon exposure to mid-ultra-violet (mid-UV) light, e.g., wavelengths between 190 nanometers (nm) and 270 nm; the photo-sensitivity of such a fiber can be enhanced by hydrogen loading. Lasers for altering the refractive index of fibers that span the above mid-UV wavelength range include ArF excimer lasers with a laser output at 193 nm and the fourth harmonic of a 1064 nm Nd:YAG laser at 266 nm.
It has been believed that the mid-UV light dislodges electrons at germanium oxygen deficient centers (GODC) to cause the change in the index of refraction. Exposing a germanium-doped fiber to mid-UV light that varies in intensity periodically in space creates a corresponding spatially varying pattern of refractive index in the fiber. Such a spatially varying index of refraction is referred to as a refractive-index grating.
Methods of generating the desired light pattern can be distinguished according to whether or not they rely on interference. Methods not employing interference rely on amplitude masks. For example, a photoresist or metal amplitude mask can be photolithographically defined on the cladding of a waveguide or a fiber, the coating of which has been removed over the region where a grating is to be formed. However, diffraction effects limit the effectiveness of the amplitude mask, especially when applied to short-period gratings. In addition, the fine structure of the amplitude mask defining the dark regions can be burned off by absorbed laser energy.
Alternatively, a single slit can be stepped across the length of fiber in which the Bragg filter is to be defined. Such a method is disclosed in U.S. Pat. No. 5,105,209. The method has been extended recently to writing long period gratings using near-UV wavelengths using argon lasers with high-coherence. (E. M. Dianov, D. S. Starodubov, S. A. Vasiliev, A. A. Frolov, O. I. Medvedkov, Paper TuCC2, Vol. 1 of LEOS'96 Annual Meeting Proceedings, pp. 374-375, 1996). Generally, the time required for the step-by-step writing is lengthy and the mechanical precision required for the stepping can be prohibitive.
More practical methods of inducing a Bragg grating take advantage of interference. A coherent laser beam can be split and the resulting beam components can be made to intersect. Due to the wave nature of light, the intersecting components will add at some locations and cancel at other locations, creating a spatially alternating pattern of light and dark.
For example, U.S. Pat. No. 4,474,427 to Hill discloses a method in which visible light is launched into the core of a fiber and reflected at an opposite end of the core. The result is a standing wave with a period corresponding to half the wavelength of the light. Through a photosensitive effect in the fiber, a refractive-index grating with this period is written into the core of the fiber. In this case, the light used was blue-green at around 480 nm. In this case, the gratings are created by two-photon absorption, corresponding to the energy associated with 240 nm light.
An important advantage of this core-launch approach is that neither the cladding nor the protective polymer coating needs to be removed for the grating to be induced. However, this method is limited to producing gratings for reflecting wavelengths close to that of the writing light. Furthermore, the core-launch approach does not provide for gratings with an arbitrary spatial variation of index amplitude and period imposed over the length of the grating itself; these include chirped and apodized gratings.
More flexibility in defining gratings can be achieved by directing interfering beams transversely. As disclosed in U.S. Pat. No. 4,807,950 to Glenn et al., two beams directed transversely into a fiber can be made to interfere. The spatially varying interference pattern creates a spatially varying refractive-index pattern. By changing the angle of the two incident light beams it is possible to vary the spatial period of the intensity pattern, which alters the reflecting wavelength of the resulting grating. The interference pattern of two light beams can be created by the use of beamsplitters and mirrors, or with a prism by the technique of Lloyd's interferometer (U.S. Pat. No. 5,377,288).
To produce an interference pattern, a laser beam must be split and then recombined. Mirror vibrations and limits on coherence length can limit the visibility of the interference pattern formed when the beams recombine. Addressing this problem, U.S. Pat. No. 5,367,588 to Hill et al. uses a phase mask in close proximity to a fiber to split the beam and direct its components so that they interfere at the fiber core. The phase mask can be a block of material with a surface relief pattern that acts as a series of beam splitters. Even low-coherence lasers, such as excimer lasers, can be used with such a phase mask. So that it transmits 245 nm light, the phase mask is formed of fused silica.
The fused silica is etched with an appropriate square-wave surface relief pattern using electron-beam lithography. When a phase mask is used, the period of the induced grating is one-half that of the surface relief pattern when the mask and core are parallel. The grating period can be increased slightly by tilting the mask slightly relative to the core. (Larger tilts result in gratings that no longer reflect light into the fiber.)
Disadvantages of this phase-mask method include the cost of the phase mask: the main expense being the cutting of the pattern using electron-beam lithography. The cost scales with the length of the phase mask; long gratings are very expensive. It is not practical to fabricate long gratings in this manner. Another expense is the fused silica which must be very pure to transmit mid-UV light.
There are also problems with obtaining an appropriate light. An excimer laser can provide the mid-UV light, but has a short coherence length. It also provides a pulsed rather than a preferred continuous output. A frequency-doubled argon laser can be used for a continuous output, but frequency doubling poses its own complications. Advances in semiconductor lasers promise better lasers in the desired frequency range, but these are not currently available. Thus, there remains room for more convenient and cost effective means for inducing gratings in an optical fiber.